This invention relates to the use of endothermic catalytic reaction apparatus operable to produce hydrogen-containing gases from hydrocarbon feedstock.
Endothermic catalytic reaction apparatus, for converting hydrocarbon feedstock to hydrogen-rich gases, is well known in the art. Commercial production of hydrogen is commonly achieved by a process known as steam reforming, that involves the endothermic reaction between a mixture of hydrocarbon feedstock and steam passed through a catalyst filled reactor tubing that is heated.
In commercial steam reformers for large-scale production of hydrogen from hydrocarbon feeds, endothermic heat is commonly supplied by the combustion of carbonaceous fuel and oxidant in a diffusion or turbulent flame burner that radiates to the refractory walls of a combustion chamber, thereby heating them to incandescence, and providing a radiant source for heat transfer to a tubular reaction chamber. Uniform radiation to the surfaces of the tubular reaction chamber is essential since excessive local overheating of the tube surface can result in mechanical failure. In large-scale commercial steam reformers, mal-distribution of heat within the furnace chamber is minimized by providing large spacing between the individual reactor tubes, the furnace walls, and the burner flames. However, for small-scale catalytic reaction apparatus that is uniquely compact, such as for the production of hydrogen for small fuel cell applications, special design features are needed to prevent tube overheating.
U.S. Pat. No. 4,692,306 to Minet and Warren describes a compact reformer comprising an annular reaction chamber concentrically disposed around an internal burner chamber containing a vertically disposed cylindrical radiant burner that uniformly radiates in the radial direction. A uniform radiation pattern to a concentrically disposed annular reaction chamber that surrounds the radiant burner, is provided, thereby avoiding the problems with flame impingement and local overheating of tube surfaces that are associated with the use of diffusion or turbulent flame burners in compact reformer apparatus.
However, there are practical limitations regarding the use of an annular reaction chamber for small-scale reformers having hydrogen production rates of less than about 1500 SCFH. It is well known that the heat transfer coefficient of gaseous reactants contained within an annular reaction chamber is directly related to the velocity of the gaseous reactants within the annular space. In order to limit the reaction chamber wall temperature, the velocity of gaseous reactants within the annular space must be sufficiently high to absorb the radiant heat flux that impinges on the reaction chamber tube walls. However, for very small-scale reformers, this requires that the width of the annular reaction chamber space be small. It is common practice in the art to limit the maximum diameter of the catalyst particles packed within an annular space to less than 20 percent of the width of the annular space in order to ensure that the catalyst is evenly distributed within the reaction chamber and to prevent gas channeling along the walls of the reaction chamber. However, for an annulus having a small width dimension, this requires use of catalyst particles of particularly small diameters thereby resulting in an undesirably high pressure drop through the catalyst bed.
The benefits of a flameless radiant burner for use in compact catalytic reaction apparatus of annular reaction chamber geometry are known. For small-scale reformer applications, a tubular reaction chamber geometry is preferred over annular reaction chamber geometry in order to simultaneously achieve high heat transfer coefficients and low pressure drops within the reaction chamber.
There is need for a compact endothermic catalytic reaction apparatus as embodied in the present invention to achieve the objects of compact design, while avoiding the problems of flame impingement, excessive reaction chamber wall temperatures, and excessive reaction chamber pressure drop by application of a tubular reaction chamber that is heated by the radiant burner. The tubular endothermic reaction chamber as disclosed herein employs a combination of catalyst particle sizes and reactant mass velocities to control the reactor pressure drop and the maximum reaction chamber tube wall temperature within certain needed limits; and the radiant burner is operated at specific ranges of combustion intensity and excess air to control surface temperature of the radiant burner within certain needed limits. The present invention extends the practical range of tubular endothermic reaction chamber geometry that can be used in combination with radiant burners for converting hydrocarbon feedstock to useful industrial gases.
It is the general object of this invention to provide a novel endothermic catalytic reaction apparatus for the production of industrial gases from a hydrocarbon or methanol feedstock that is simultaneously compact, thermally efficient, has improved life expectancy and low pressure drop, and is particularly well suited for the small scale generation of useful gases for fuel cell applications in the range of 1 k W to 50 k W.
In the present invention, a compact burner chamber employing a radiant burner assembly is configured to distribute radiant energy along the axial length of a tubular reaction chamber. In one embodiment, the radiant burner assembly comprises a woven metal fiber attached to a support structure that permits the efflux of fuel and oxidant from the burner core to the outer surface of the metal fiber. The properties of the metal fiber stabilize the combustion in a shallow zone proximal to the outer surface of the metal fiber. The combustion reaction heats the metal fiber to incandescence and provides a source of radiant energy that is transferred to the reaction chamber. In another embodiment, the radiant burner assembly comprises a porous ceramic fiber burner that accomplishes the same object by serving as a radiant source of energy.
The metal fiber of the burner typically consists essentially of an alloy containing principally iron, chromium, and aluminum and smaller quantities of yttrium, silicon, and manganese having extended life at operating temperatures up to 2000xc2x0 F.
In one embodiment, the tubular reaction chamber has U-shape, and is sometimes referred to as a hairpin tube, which is substantially filled with catalyst, the tube extending into and out of the combustion chamber for gaseous flow through. The radiant burner axis is preferably vertically disposed within the combustion chamber and oriented parallel to the axis or axes of the U-tube reaction chamber. The active radiant surface of the cylindrical radiant burner assembly is defined by a geometric arc that bisects the cylindrical assembly so as to maximize the flux of radiant energy that is directed to the surface of the U-tube reaction chamber. In this embodiment, the center to center spacing between the radiant burner and the U-tube reaction chamber, and the radiation angle of the radiant burner are simultaneously controlled, or configured for high efficiency of heat transfer.
In a third embodiment, the tubular reaction chamber comprises a helical coil that is substantially filled with catalyst and has inlet and outlet portions that pass into and out of the combustion chamber. The helical coil is wrapped to form turns at specific lead angles, so that the coil free area is in the range of 50% to 75%, wherein the free area is defined by the ratio of the free area between helical tube conduits or turns and the cylindrical surface that bisects the helical coil circle or cylinder. The radiant burner axis is typically vertically disposed within the combustion chamber and the cylindrical radiant burner is located at the center of the helical coil. In this embodiment, the active radiant surface of the cylindrical radiant burner assembly is defined by a 360-degree arc.
In each embodiment, the radiant burner is operated at a combustion intensity and an excess air ratio that is carefully controlled to limit the radiant burner surface temperature to less than 2000xc2x0 F., and preferably in the range of 1500xc2x0 F. to 1900xc2x0 F., in order to provide extended life for the radiant burner.
In each embodiment, the catalyst particle diameters and reactant mass velocities are carefully controlled to simultaneously limit the reactor pressure drop to less than 8 psi, and preferably in the range of 2 psi to 4 psi in order to limit the delivery pressure required for the hydrocarbon feeds, and to limit the reaction chamber tube wall temperatures to less than 1600xc2x0 F., and preferably in the range of 1300xc2x0 F. to 1500xc2x0 F., in order to allow extended life of the tube using relatively inexpensive tube alloys.
In each embodiment, a portion of the combustion chamber is configured to form an annular convective chamber to enhance heat transfer from the combustion products to the tubular reaction chamber.
A further object is to provide endothermic catalytic reaction apparatus, comprising
a) a combustion chamber,
b) a tubular reaction chamber having two generally tubular legs extending in generally parallel, spaced apart relation within the combustion chamber,
c) catalyst within said reaction chamber for reacting with a hydrocarbon and steam received within the reactor chamber, to produce hydrogen and carbon dioxide,
d) a radiant burner within the combustion chamber and extending in generally parallel relation to at least one of said legs, said burner spaced from said legs,
e) said two legs having axes, and said burner having an axis which is spaced in offset relation to a plane defined by said leg axes.
In yet another embodiment, the tubular reaction chamber comprises a straight tubular outer conduit concentrically disposed around an inner conduit. Catalyst is contained in the annular space between the outer conduit wall and the inner conduit wall. The tubular reaction chamber is configured so that the flow of reactant gas is directed longitudinally through the annular catalyst space in one direction and returns down the inner conduit space in the opposite direction. A portion of the tubular reaction chamber extends into the combustion chamber. One end of the tubular reaction chamber, containing both an inlet means that is in communication with the annular catalyst space and an exit means that is in communication with the inner conduit space, extends outside of the combustion chamber. A radiant burner is oriented to direct a flux of radiant energy to the surface of the outer conduit of the tubular reaction chamber. If a multiplicity of such tubular reaction chambers are used, they can be oriented concentrically around a centrally disposed radiant burner that uniformly radiates in a 360 degree arc. The radiant burner may consist of metal fiber material, or ceramic fiber material.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: